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    <meta charset="UTF-8">
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    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
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<script>
    /**
     *在开头unshift(被添加元素)  在末尾push(被添加元素) 
     返回值为新的长度
     */
    var currentday = "星期天";
    var daylist = [currentday];
    daylist.unshift("");
    daylist.push("");

    var arr1 = ["hello","world"];
    var arr2 = ["hello","js"];
    var arr3 = ["arr3"];
/**
 *arr.concat(arr1,arr2); 
 * 会将arr，arr，arr2合并，得到一个新的
 * 数组，但是不会对arr产生影响
 * 
 */
    var arr4 = arr1.concat(arr2,arr3);
    console.log(arr1);
    console.log(arr4);
</script>
</html>